1. Technical Field
The present invention relates to a semiconductor device having a bleeder resistor composed of resistors formed by polycrystalline silicon.
2. Description of the Related Art
In a semiconductor integrated circuit, a diffused resistor or a polycrystalline silicon resistor is used. The diffused resistor is made from a single crystalline silicon semiconductor substrate into which impurities of the opposite conductivity-type to that of the semiconductor substrate are implanted. The polycrystalline silicon resistor is formed of polycrystalline silicon into which impurities are implanted. The polycrystalline silicon resistor has, in particular, advantages in small leakage current due to insulating films surrounding the resistor and in a high resistance brought by defects existing at grain boundaries, leading to wide use in semiconductor integrated circuits.
FIGS. 3A and 3B are a schematic plan view and a schematic sectional view of a conventional polycrystalline silicon resistor circuit, respectively. The polycrystalline silicon resistor is produced by implanting p-type or n-type impurities to a polycrystalline silicon thin film deposited on an insulating film by low pressure chemical vapor deposition (LPCVD) or the like, and then processing the resultant into a resistor shape with a photolithography technology. Impurity implantation is performed for determining a resistivity of the polycrystalline silicon resistor. Depending on a desired resistivity, a concentration of the p-type or n-type impurities to be implanted ranges from 1×1017/cm3 to 1×1020/cm3. Further, at each terminal on both sides of the resistor, a contact hole and a metal wiring are formed to obtain the potential thereof. A satisfactory ohmic contact between the polycrystalline silicon and the metal wiring layer at the terminal requires selective implantation of impurities at a high concentration of equal to or more than 1×1020/cm3, by using a patterned photo resist, into a portion of the polycrystalline silicon corresponding to the terminal of the resistor.
The resistor using the polycrystalline silicon is structured, as illustrated in the schematic plan view of FIG. 3A and the schematic sectional view of FIG. 3B, to include a polycrystalline silicon 103 made of a low concentration impurity region 104 and a high concentration impurity region 105, which is formed on an insulating film 102 on a semiconductor substrate 101. A potential is obtained from a metal wiring 107 through a contact hole 106 formed above the high concentration impurity region 105.
Further, as illustrated in FIG. 3B, a metal is placed on the above-mentioned polycrystalline silicon 103 made of the low concentration impurity region 104 and the high concentration impurity region 105 so as to prevent hydrogen, which affects a resistance of the polycrystalline silicon, from diffusing into the polycrystalline silicon in a semiconductor process. The polycrystalline silicon is composed of grains having relatively high crystallinity and a grain boundary between the grains which has low crystallinity, that is, a high level density. The resistance of the polycrystalline silicon resistor is mostly determined by electrons or holes, which serve as carriers, trapped by a large number of levels existing at the grain boundary. However, when hydrogen having a high diffusion coefficient is generated in a semiconductor manufacturing process, the hydrogen easily reaches the polycrystalline silicon to be trapped by the levels, thus varying the resistance. Examples of the hydrogen generating process include a sintering step in a hydrogen atmosphere after metal electrode formation and a forming step of a plasma nitride film using an ammonia gas. Covering the polycrystalline silicon resistor with the metal wiring layer may suppress the variation of the resistance of the polycrystalline silicon due to the hydrogen diffusion.
The method for stabilizing the resistance of the polycrystalline silicon is disclosed in JP 2002-076281 A, for example.
However, the method for stabilizing the resistance of the polycrystalline silicon has the following problem. That is, there is a problem in the semiconductor manufacturing process that the metal on the polycrystalline silicon is susceptible to factors other than hydrogen which affect the polycrystalline silicon, such as heat, stress, and charging due to plasma. Those factors affect the polycrystalline silicon through the metal thereon, resulting in the variation of the resistance.
Further, the resistance may be changed due to a difference between a potential of the metal provided above and a potential of the resistor provided below. This is conceivably because, by using the resistor formed of the polycrystalline silicon into which a large number of impurities are implanted, the above-mentioned potential difference causes the impurity concentration to be changed in the polycrystalline silicon resistor. Therefore, the way of obtaining the potential from the metal portion provided above also affects the variation of the resistance.
In a circuit using a bleeder resistor, for example, a circuit such as a voltage detector or a voltage regulator, an output current/voltage value is determined by a ratio of bleeder resistances. However, when the resistances change even slightly, a resistance ratio accuracy of a resistor group is reduced and hence a desired value for the output current/voltage value may not be obtained. This leads to a lowered yield especially in a case of a product requiring precision.